1. Field of the Invention
The field of the invention pertains to the prevention and detection of the unauthorized tampering or counterfeiting of valuable documents. In particular, the field of the present invention pertains to the use of thermo-activated pantographs and validation marks to protect against the unauthorized alteration and counterfeiting of documents.
2. The Related Art
Presently known approaches for recognizing altered, duplicate or counterfeit copies of original documents have generally sought to prevent alteration, duplication or counterfeiting of documents by physical alteration, electronic scanning or xerographic photocopying by printing information on the document in specially designed inks or other printing materials so that the printed information can be viewed on the original, but due to the unique color, texture, or reflective properties of the printed information on the altered or counterfeited copy, will be readily discernable from the original.
For example, U.S. Pat. No. 4,066,280 to LaCapria describes a document upon which is printed a specularly reflective coloring material such as powdered aluminum, which is not accurately reproduced by color copiers. The duplicate image will appear in different colors than on the original.
Similarly, U.S. Pat. No. 4,988,126 to Heckenkamp et al. describes an original document having surface relief in the form of embossed characters. A luminescent substance is formed into raised or depressed areas of the surface relief. The reflective properties of the surface relief render the original readily discernable from a photocopy which lacks the surface relief.
U.S. Pat. No. 4,082,426 describes retroreflective sheet materials formed of a monolayer of microsphere-lenses overlaying a specularly reflective layer coated over a polymeric material. A transparent image layer of varying thickness permits light rays to be transmitted to and reflected by the specularly reflective layer behind the image layer. The varying thickness of the image layer and the spacing between the specularly reflective layer and the microsphere-lenses changes the reflective characteristics of the sheeting, so that markings on the sheet are visible only from certain angles under retroreflective viewing conditions.
U.S. Pat. No. 4,892,385 to Webster, Jr. et al. describes an authenticating device which can be bonded to the surface of a document to identify an original document.
Another approach has been to provide specially manufactured copy-resistant paper upon which information of any kind can be printed using conventional processes and inks. For example, U.S. Pat. No. 4,867,481 to Gundjian describes copy-resistant paper having a two-color grid-like pattern printed over its surface, with each color having the same spectral profile but different spectral response. U.S. Pat. No. 4,303,307 to Tureck et al. describes a paper substrate coated with specially sized and spaced beads which break up incident light emitted by a photocopier. U.S. Pat. No. 5,093,184 to Edwards describes security paper having elongated metallic elements embedded in the paper.
Yet another approach has been to provide specially designed inks or other printing materials having different or unique color or reflective properties. For example, U.S. Pat. No. 5,271,645 to Wicker describes a color-copier resistant pigment consisting of print stuff mixtures obtained by mixing commercially available pigments with fluorescence compound.
U.S. Pat. No. 4,869,532 to Abe et al. describes a print produced by printing or coating an infrared reflective coloring agent and another printing ink containing an infrared absorptive coloring agent in combination on a base material, to produce visually-recognizable information along with other information recognizable with the aid of infrared lighting.
U.S. Pat. No. 4,025,673 and U.S. Pat. No. 3,887,742 to Reinnagel describe prevention of photocopying by selection of different color or color filter combinations for the text and background.
U.S. Pat. No. 4,175,776 to Ranauro describes a document in which the text and background are characterized by different optical reflectivities for incident visible light and which are substantially non-absorbing with respect to incident light having wavelengths within the response spectrum of color xerographic copying machines. When the document is photocopied, the incident light of the photocopier produces a uniform reflected pattern over the indicia which causes the indicia to "drop out" of the copy.
U.S. Pat. No. 4,522,429 to Gardner et al. discloses a document upon which text is printed upon colored paper having a reflection spectral response of less than about ten percent for light of below 600 millimicron wavelength, so that the color is sufficiently contrasting with the text to be visible when viewed under white light, but cannot be successfully photocopied.